Apparatus for testing microphones



Feb. 12, 1946. H, J, HOULGATE ETAL I 2,394,613

APPARATUS FOR TESTING MICROPHONES Filed .July .27, 1942 41% MAA/mg@ m4,

Patented Feb. 12, 1946 2,394,613 APPARATUS Foa TESTING Mlonornouusu Henry John Houlgate and Philip David Rhodes Marks, London, England, assignors to Guy R. Fountain Limited, London, England, a British Company Application July Z7, 1942, Serial No. 452,526 In Great Britain July 31, 1941 8 Claims.

The present invention relates to apparatus for testing microphones and has for its object to provide a sound generator for testing a close talking The mouth as a source of sound is of small dimensions compared with the Wavelength in air of the frequencies at which maximum energy is radiated, [and thus is virtually a source working into an inductive load of low value and, in fact, simulates the radiation impedance presented to a piston in a baiile at a frequency Whose wavelength is large compared with the dimensions of the piston. The position is further complicated by the fact that a close-talking microphone has a serious effect upon the impedance presented to the mouth, and this effect must be simulated in an artificial mouth if accurate test results are to be obtained.

At the commencement of most Words the mouth produces a puff of air which, when received on a microphone positioned close to the mouth, exerts a uni-directional pressure on the diaphragm for a short period of time. In the case of a carbon microphone, this impulse serves to agitate the granules and leave them in a more sensitive condition than if the speech were applied without the application of this impulse.

The present invention consists in a test apparatus for microphones comprising a sound generator which is coupled to a tube which is open at its other end, the open end being of small diameter compared with the wavelength of the frequency used so that the emitted sound has a spherical Wave front. A feature of the invention consists in driving the source at resonance in such a manner that the sound field outside the source has a very high velocity component, that is, the acoustic impedance presented to the source with no microphone in position is low, whereby an air pressure is exerted on the microphone under test which simulates the puff of air produced `from the mouth.

'I'he sound system may be energised from an external source of oscillation or may be selfexcited.

In order that the invention may be more clearly understood an embodiment thereof will now be described, by way of example, with reference to the accompanying drawing', in which Fig. 1 shows the construction of the acoustic part of the apparatus.

Fig. 2 shows a circuit diagram.

The embodiment of the invention shown in the 'drawing provides a test apparatus which approximates very nearly to the conditions existing in the human mouth. 'I'he sound generator comprises a loud speaker drive unit i of the small diaphragm type, which is usually loaded with a horn, which is energised at a desiredfrequency from any suitable electric oscillation generator. The loud speaker drive unit illustrated is of the type described in the specification of British Patent No. 519,625, but this forms no part of the invention and the drive unit may be of any suitable construction. The drive unit is coupled to a tube 2 in any suitable Way, for example, by means of a union nut t which clamps a 'collar 5 at the end of the tube 2 to the neck 3 of the loud speaker unit, a rubber washer 6 being interposed between the end of the collar 5 and the neck 3.

The diameter of the tube 2 is made small compared with the wave-length of the frequency used. In one particular case with a frequency of 400 cycles per` second, the inside diameter of the tube was 17e". The length of the tube is adjusted so that resonance occurs with a velocity antinode at the open end. In one such apparatus which was constructed, in which the input was adjusted to obtain an output loudness level of approximately -120 phons at a point about 1" from the open end of the tube, the velocity component was so large that a jet of air was formed outside the tube which was sufficient to extinguish a Vlighted match held four or iive inches from one end of the tube.

The microphone or microphone unit 1 to be tested is located at a fixed distance from the end of the tube in any suitable way, preferably by means of a very openly constructed supporting member 8 carried by a collar 9 which is mounted on the end of the tube 2 and is slidable longitudinally thereon to adjust the distance at which the ymicrophone is located from the end of the tube, the collar being secured in the adjusted position by means of the set screw l0.

It will be noted that the end of the tube 2 adjacent the microphone is freely open to atmosphere, so that there is no closed column of air coupledvto the microphone, such as might restrict the movement of its acoustic members. It will be further understood that the distance of the microphone from such open end of the tube is preferably approximately equal to the normal iations may be effected by adjusting the microphone-supporting member on the tube.

Fig. 2 shows a, circuit arrangement of the test apparatus. The oscillations of the desired frequency are produced by the mains energised oscillator II. The output from the oscillator II is fed through the power amplifier 14, and an adjustable level setting device ifto the loud speaker unit I. The output which is fed to the loud speaker unit is measured on the vvoltmeter le. g

The low tension accumulator I2 lfeeds current to the microphone l, underv test. The voltage of battery I2 is indicated on the voltmeter I3 and is maintained within a predetermined voltage range,'being re-charged as soon as its voltage falls below a predetermined minimum value. The current which flows through the microphone'is measured on the ammeters I1, the microphone output being fed through a microphone transformer I8 and load I9 and being measured on the output voltmeter' 2li'.V The microphone being tested is assumed to be a carbon microphone of the differential type such as described in the specification of British Patent lNo. 507,183 and three ammeters are provided for measuring the current owing through the front electrodes, the rear electrodes, and all the electrodes together, respectively. In the testing of microphones of different types, of course, all the ammeters may not beV necessary. Preferably the oscillator and power amplifier or the voutputs therefrom are switched on automatically on depressing the push button 'Ia on the microphone Awhich permits current to ilow through the microphone. The battery current may actuate relays for this purpose. Alternatively, a separate/switch may be provided for switching on the oscillator or connecting its output to the drive unit, which switch may, for example, operate autorsnatically when the microphone is placed imposition-in the test apparatus/ In operating the equipment, the level setter I is adjusted to obtaina predetermined indication on the voltmeter I6, the microphone under test being passed as good or being rejected depending upon Whether the output indicated on the voltmeter is within the predetermined range drilled in the wall ofv the. tube and a microphone coupled thereto,jthe output from which is con-` netted to the input of the ampiine a feeding the drive unit I. Phase-correcting networks may be necessary to bringabout the required conditions to give acoustic oscillations in the tube at ensure that the microphone used has a suciently high acoustic impedance' not to disturb seriously the conditions in the tube at Vthe pressure antinode. For example, a small piezo-electric element could be mounted inside the tube or, as shown in Fig. 1, a microphone 2I of low acoustic impedance could be coupled to the tube through a high acoustic resistance, such as a length .of capillary tubing 22. f i

The output from the microphone 2| is fed back to the drive unit I .through the correcting network 23 and the vamplifier I4.

in an alternative methodV for self-exciting the apparatus, a very small velocitymicrophone may be located at the mouth of the tube.

In practice, due to the presence of the complicated acoustic impedance of the drive unit. the tube will tend to oscillate at several frequencies if the exciting microphone has a uniformresponse characteristic. The oscillations may be confined to the ldesired frequency by .adjusting either the amplitude-frequency vresponse of the microphone to be a maximum at that frequency or by adjusting the phase-frequency characteristic of the microphone so that only at the desired frequency is the phase correct for oscillations to occur.

Although particular embodiments according to the invention'have been described infsome detail, it is to be understood that various modifications may be'made Without departing from the spirit of the invention, which consists broadly in an artificial mouth for testing microphones,

' which produces sounds closely simulating those ing such that resonance occurs with a velocity antinode at the open end of the' tubeandmeans for supporting the microphone'undr test in front of and spaced away from said open end of the tube.

2. Apparatus for testing microphones compris- 591mg a sound generator which is coupled to a tube which is freely open to atmosphere at its other end, the internal diameter of the tube being small compared with the wavelength of the sounds generated and the length of the tube being such that a blast of airis produced by the sounds at the' open end of the tube-simulating the blast produced by the human mouth when speaking. l

8. Apparatus for testing microphones compris- 00 ing a sound generator which ,is coupled to a tube which is open at its other end and is of small diameter compared .with the wavelength of they frequency used sothat the emitted sound has a 5 spherical wave front, the length of the tube being the normal distancefo the human mouth when talking into the rrrierophone.y

'4. Apparatus fortesting microphones comprising a sound -generator which is coupled to va. tube whichls freely opento atmosphere at its other the desired frequency. Care must be taken to end, the internal diameter of the tube being small compared with the wavelength of the sounds and the length of the tube being such that a blast of air is produced by the sounds at the open end o the tube, and means for supporting the microphonev or microphone unit under test in front of and spaced away from the open end of the tube a distance approximating the normal distance of the human mouth when talking into the microphone.

5. Apparatus as claimed in claim 4, wherein the supporting means comprises an open framework which is carried by the end oi the tube and is adjustable longitudinally thereof. y

6. Apparatus for testing microphones com` prislng a sound generator which is coupled to a tube which is open at its other end, the open end being of small diameter compared with the wavelength of the frequency used so that the emitted sound has a spherical wave front, and means for supporting the microphone or microphone unit under test in front of and spaced away from the open end of the tube a distance approximating the normal distance of the human mouth when talking into the microphone.

7. Apparatus as claimed in claim 6, wherein a microphone is associated with the tube which picks up the sounds and feeds them back to the sound generator whereby the apparatus is seliexcitlng.

8. Apparatus as claimed in claim 6, comprising a microphone, a capillary tube connecting said microphone to a pressure antinode in the tube coupled to the sound generator, and means for feeding the output from said microphone back to the sound generator.

HENRY JOHN HOULGATE. PHILIP DAVID RHODES MARKS. 

